A new linear charging solution for CR927 battery

　　With the development of modern electronic technology, electronic devices are becoming increasingly portable and multi-functional, so their power supply batteries are also required to be lightweight and efficient. CR927 battery have gradually replaced traditional nickel-cadmium, nickel-hydrogen batteries, and lead-acid batteries due to their high energy density, excellent charge and discharge performance, and no pollution. They are widely used in modern portable electronic products.

　　Compared with other types of batteries, CR927 battery have excellent performance and also put forward higher requirements for chargers. These requirements are mainly reflected in the control of the charging process and the protection of lithium batteries. Specifically, they include larger charging current, high Accurate charging voltage, staged charging mode and complete protection circuit, etc.

　　This article discusses the design of a linear charging scheme for CR927 battery using the high-current lithium-ion battery charging chip SE9018.

　　Chip introduction

　　SE9018 is a constant current/constant voltage mode lithium-ion battery linear charging chip that uses an internal pMOSFET architecture and integrates an anti-reverse charging circuit without the need for external isolation diodes.

　　The chip's preset charging voltage is 4.2V, with an accuracy of ±1.5%. The charging current can be set through an external resistor, and the maximum continuous charging current can reach 1A. When the junction temperature of the chip is higher than 140°C due to high operating power, high ambient temperature, or poor PCB heat dissipation performance, the internal thermal feedback circuit will automatically reduce the charging current and control the chip temperature within a safe range. In order for the chip to maintain efficient working status, measures should be taken to reduce the chip operating power and chip temperature as much as possible, such as connecting a small resistor in series with the input end (lowering the input voltage), increasing the PCB heat dissipation copper foil area, and making the chip heat sink and PCB copper foil fully Contact etc.

　　SE9018 integrates a battery temperature monitoring circuit. When the battery temperature exceeds the normal range (too high or too low), the chip automatically stops the charging process to prevent battery damage due to too high or too low temperature.

　　Battery temperature monitoring is achieved by judging the TEMp terminal voltage (VTEMp). VTEMp is provided by a resistor voltage dividing network including the battery's internal NTC thermistor.

　　When VTEMp is between 45%×VCC and 80%×VCC, the chip determines that the battery temperature is within the normal range; when VTEMp<45%×VCC or VTEMp>80%×VCC, the chip determines that the battery temperature is too high or too low. ;When the TEMp terminal is connected to ground, the battery temperature monitoring function is disabled.

　　SE9018 contains two open-drain status indication output terminals CHRG and STDBY. When the circuit is in the charging state, the CHRG terminal is set to low level and the STDBY terminal is in a high-impedance state; when the battery is fully charged, the CHRG terminal becomes a high-impedance state. , STDBY terminal is set to low level. When the battery temperature monitoring function is in normal use, if the chip is not connected to the battery or the battery temperature exceeds the normal range, both the CHRG and STDBY terminals are in a high-impedance state; when the battery temperature monitoring function is disabled, if the chip is not connected to the battery, the STDBY terminal is Low level, CHRG terminal outputs pulse signal.

　　Other functions of SE9018 include manual shutdown, undervoltage lockout, automatic recharging, etc.

　　A typical SE9018-based lithium-ion battery charging circuit is shown in Figure 3. When the CE terminal is high, SE9018 works normally.

　　Figure 3SE9018 typical application circuit

　　1. Setting of charging current

　　The charging current Ibat during the constant current charging process is set by the resistor Rprog between the pORG terminal and the GND terminal. The relationship between the resistance values of Ibat and Rprog is:

　　Formula 1

　　For example, if you want to get a constant charging current of 1A, according to Formula 1, you can get Rprog=1200Ω.

　　2. Battery temperature monitoring circuit settings

　　The setting of the battery temperature monitoring circuit is mainly to set R1 and R2. It is assumed that the resistance of the NTC thermistor at the lowest operating temperature is RTL and the resistance at the highest operating temperature is RTH (the data of RTL and RTH can be found in the relevant battery manual Or obtained through experiments), then the resistance values of R1 and R2 are:

　　Formula 2

　　Formula 3

　　In practical applications, if only high-temperature protection is required and low-temperature protection is not required, R2 can be removed. At this time, the resistance of R1 is:

　　Formula 4

　　3. Manual shutdown setting

　　During the charging process, SE9018 can be put into shutdown state at any time by setting the CE terminal to low level or removing Rprog (pROG terminal is floating). At this time, the battery leakage current drops below 2uA and the input current drops below 70uA.

　　4. Undervoltage lockout state

　　If the input voltage VCC is lower than the undervoltage lockout threshold or the difference between VCC and the battery voltage Vbat is less than 120mV, the SE9018 is in the undervoltage lockout state.

　　When the chip is in shutdown state or under-voltage lockout state, both the CHRG terminal and STDBY terminal are in a high-impedance state.

　　5.Normal charging working cycle

　　When each input terminal of the SE9018 and the battery are in normal status, the charging circuit enters the normal charging cycle. This cycle includes four basic operating modes: trickle charging, constant current charging, constant voltage charging, charging end and recharging.

　　If the battery voltage Vbat is lower than 2.9V, the charging circuit enters the trickle charging mode, and the charging current is one-tenth of the constant charging current (if the constant charging current is set to 1A, the trickling charging current is 100mA) , the trickle charging state will be maintained until the battery voltage Vbat reaches 2.9V. The trickle charging mode is mainly to avoid damage to the internal structure of the battery caused by large current surges when the battery voltage is too low.

　　When the battery voltage is higher than 2.9V but less than the preset full voltage of 4.2V, the charging circuit is in constant current charging mode. As mentioned above, the charging current is determined by Rprog.

　　When the battery voltage reaches 4.2V, the charging circuit enters the constant voltage charging mode. At this time, the BAT terminal voltage is maintained at 4.2V, and the charging current gradually decreases. The main function of this process is to reduce the impact of the battery's internal resistance on the full charge voltage, so that the battery can be charged more fully.

　　When the charging current is reduced to 1/10 of the constant current charging current, the charging circuit stops charging the battery and enters a low-power standby state. In standby mode, SE9018 will continuously monitor the battery voltage. If the battery voltage drops below 4.05V, the charging circuit will charge the battery again.

　　6. Indicator status

　　Table 1:

　　7. Circuit compatible with USB power supply and adapter power supply

　　At the same time, the SE9018 chip can be used to implement a charging circuit suitable for USB power supply and adapter power supply. The circuit diagram is shown in Figure 4.

　　When using the USB power supply, the pMOS and NMOS gates are pulled down to low potential, the pMOS is turned on, the USB power supply supplies power to the SE9018, and the SCHOTTKY diode prevents the USB end from leaking to the adapter end. NMOS is turned off, Rp1 is disconnected, Rprog=2.4kΩ, and the constant charging current is 500mA.

　　When using a 5V adapter for power supply, the pMOS and NMOS gates are at high potential, and the pMOS is turned off to prevent the adapter end from leaking to the USB end. The 5V adapter voltage supplies power to the SE9018 through the SCHOTTKY diode. NMOS is turned on, and Rp1 is connected to the circuit. At this time, Rprog is Rp1 and a 2.4kΩ resistor in parallel. By setting Rp1, a constant current charging current greater than 500mA can be achieved.

　　Conclusion of this article

　　Properly implementing battery charging in today's portable products requires careful design considerations. This article discusses a smart high-current lithium-ion battery linear charging solution. The SE9018 chip used has the characteristics of fast charging speed, strong battery protection function, and fewer peripheral components. Moreover, the chip is also suitable for USB power supply and adapter power supply. It is a more practical intelligent high-current lithium-ion battery charging chip.

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